In the process of drilling an oil and gas well, drilling fluids are commonly used to perform a number of functions. In addition to cooling the bit, providing lubrication, stabilizing fluid loss, and counterbalancing pressure, drilling fluids are circulated to remove pieces of rock chips, gravel, and debris (known as “cuttings”) from the wellbore while it is being drilled. Drilling fluid is pumped down the drill string, cuttings are suspended in the fluid and carried out of the well through the annulus between the drill string and the wellbore.
Proper hole cleaning is a requirement in all wells, but it becomes particularly important in drilling highly deviated wells, horizontal wells, and extended reach wells. In drilling such wells, gravity causes cuttings and other debris to build up along the bottom side of the wellbore and form deposits known as “cuttings beds.” Drilling fluids are generally ineffective for removing these cuttings, which may cause formation hole fill ups, decreased bit life, differential sticking, decreased rate of penetration and other problems.
Mechanical and chemical solutions have been proposed to address the need for cuttings removal in wells that pose hole-cleaning challenges. Chemical solutions include varying the drilling fluid properties and rates or adding special additives, which enhance the ability of the fluid to transport the cuttings. Usually drilling operations must be stopped while the fluid is added or circulated through the well.
One simple mechanical solution is to rotate the drill pipe to agitate the fluid and mobilize the cuttings. This method is rather ineffective for cleaning large amounts of accumulated cuttings and has limitations when applied in non-rotating drilling operations (e.g. coiled tubing). Another mechanical solution involves attaching an oscillator or vibrator to the end of the drilling apparatus and activating the oscillator or vibrator to loosen the debris from the wall of the well. A drawback of this method is that it is only effective for cleaning cuttings in very close proximity to the tool. Donwhole cleaning tools with fixed external blades have also been developed as a mechanical approach to hole cleaning. Such tools are use by reciprocating (alternatively raising or lowering) the drill string to assist in the removal of cuttings beds. These cleaning tools are not practical in non-accumulating cuttings areas (outside of the cutting bed) because the fixed blades increase the torque and drag on the drill string resulting reduction in circulation of the drilling fluid and overall cleaning effectiveness. In addition, moving the drill string up and down risks damaging the tool.
A recent development in the area of hole cleaning is the use of the principle of cavitation for removing cuttings, dirt, parafins, asphaltenes, and other debris. Cavitation generally refers to the formation and instantaneous collapse of innumerable tiny vapor bubbles within a fluid subjected to rapid and intense pressure changes. A liquid subjected to a low pressure (tensile stress) above a threshold ruptures and forms vaporous cavities. When the local ambient pressure at a point in the liquid falls below the liquid's vapor pressure at the local ambient temperature, the liquid can undergo a phase change, creating largely empty voids termed cavitation bubbles. Fluid pumped through the tool drives a mechanical process that induces cavitation, and a flare of bubbles is released. The combined effects of the flow impact, the suction effects of the decaying bubble flare, and the implosion shock waves of the cavitation are effective to mobilize and remove debris that may be trapped in the wellbore.